CN1325685C - Nano-composite martensitic steels - Google Patents

Abstract

Carbon steels of high performance are disclosed that contain dislocated lath structures in which laths of martensite alternate with thin films of austenite, but in which each grain of the dislocated lath structure is limited to a single microstructure variant by orienting all austenite thin films in the same direction. This is achieved by careful control of the grain size to less than ten microns. Further improvement in the performance of the steel is achieved by processing the steel in such a way that the formation of bainite, pearlite, and interphase precipitation is avoided.

Description

nanocomposite martensitic steel

field of invention

The present invention relates to steel alloys, especially those steel alloys with high strength, high toughness, high corrosion resistance and high cold formability, and also relates to steel alloy processing techniques capable of forming steel microstructures.

Background of the invention

The following US patents describe high-strength, high-toughness, cold-formable steel alloys having a multiphase martensitic and austenitic microstructure, which are hereby incorporated by reference in their entirety:

4170497 (Gareth Thomas and Bangaru V.N.Rao), issued October 9, 1979, application filed August 24, 1977;

4170499 (Gareth Thomas and Bangaru V.N.Rao), issued October 9, 1979, application filed September 14, 1978, a continuation-in-part of the above application filed August 24, 1977;

4619714 (Gareth Thomas, Jae-Hwan Ahn and Nack-Joon Kim), issued October 28, 1986, application filed November 29, 1984, is a continuation-in-part of an application filed August 6, 1984 ;

4671824 (Gareth Thomas, Nack-Joon Kim and Ramamoorthy Ramesh), issued June 9, 1987, application filed October 11, 1985;

6273968 (Gareth Thomas), issued Aug. 14, 2001, application filed Mar. 28, 2000.

The microstructure plays a key role in the formation of the properties of special steel alloys. Therefore, the strength and toughness of the alloy not only depend on the selection of the type and amount of alloying elements, but also on the crystalline phases and their arrangement in the microstructure. Alloys for use in certain environments require high strength and toughness, and often require some combination of conflicting properties, since certain alloying elements that favor one property may be detrimental to another.

The alloys described in the patents listed above are carbon steel alloys whose microstructure consists of alternating laths of martensite and thin films of austenite. In some cases, fine grains of carbides produced by autotempering are dispersed in the martensite. The arrangement of one lath phase separated by another thin film phase is called a "displaced lath" structure, which is formed by heating the alloy to the austenite range and then cooling the alloy below the martensite onset temperature Ms A temperature range at which austenite transforms into a structure separated by laths of martensite separated by films of untransformed but stabilized austenite, said martensite initiation temperature being the initiation of the martensite phase forming temperature. At this point standard metallurgical processes such as casting, heat treatment, rolling and forging are performed to obtain the desired product shape and refine the alternating arrangement of laths and films. This microstructure is better than the twinned martensite structure because of the greater toughness of the lath structure. These patents also introduce that during the cooling process, the excess carbon in the lath area will undergo the so-called "self-tempering" phenomenon, forming cementite (iron carbide, Fe ₃ C) and precipitation. According to the patent '968, the self-tempering Fire can be avoided by limiting the choice of alloying elements so that the martensite initiation temperature Ms is 350°C or higher. In some alloys, the carbides produced by autotempering increase the toughness of the steel, while in other alloys the carbides limit the toughness.

The misaligned lath structure forms a high strength steel that is both tough and ductile, properties needed to prevent crack propagation and adequate formability for the successful manufacture of engineering components from steel. One of the most effective ways to achieve the desired strength and toughness is to control the martensitic phase so that dislocated lath structures form rather than twinned structures, while thin films of retained austenite enhance ductility and formability. To obtain this dislocated lath microstructure rather than the less favorable twinned structure requires careful selection of the alloy composition, which in turn affects the M _s value.

The stability of austenite in the dislocated lath microstructure is a factor in the alloy's ability to retain its toughness, especially when the alloy is exposed to harsh mechanical and environmental conditions. Under certain conditions, austenite is unstable at temperatures above 300°C and will transform into carbide precipitates, making the alloy relatively brittle and less able to withstand mechanical stress. This instability is one of the problems that the present invention sets out to solve.

Summary of the invention

It has now been found that carbon steel alloy grains having the above-described dislocated lath microstructure tend to form multiple regions in a single grain structure with different orientation directions of the austenite film. During transformation strain formation accompanying the formation of dislocated lath structures, different regions of the austenitic crystal structure undergo shearing in different planes of the face centered cubic arrangement (characteristic of austenite). While not being bound by this explanation, the inventors believe that this results in the formation of the martensitic phase due to the shear action in different directions throughout the grain, thereby forming domains as described below: The austenite films are arranged at a common angle within the region, but at different angles between adjacent regions. Due to the austenitic crystalline structure, as many as four regions can result, each with a different angle. This confluence of these regions forms a crystalline structure of limited stability for the austenitic film. Note that the grains themselves are enclosed by austenite shells at their grain boundaries, whereas the regions between grains of different austenite film orientations are not enclosed in austenite.

It has also been found that said dislocated lath-structured martensite-austenite grains with a single-oriented austenite film can be obtained by limiting the grain size to 10 micrometers or less, and it has also been found that said grains Carbon steel alloys are more stable when exposed to high temperatures and large mechanical strains. Accordingly, the present invention relates to carbon steel alloys comprising dislocated lath microstructural grains, each grain containing a single-oriented thin film of austenite, ie each grain is a single variable of the dislocated lath microstructure.

The invention also relates to a method of preparing such a microstructure by holding the alloy composition at a temperature such that the iron is completely in the austenitic phase and all alloying elements are in solid solution (austenitization) , which is then held at a temperature just above its austenite crystallization temperature, forming small grains 10 microns or less in diameter. Afterwards, the austenite phase is rapidly cooled to the martensite initiation temperature, and part of the austenite is transformed into a dislocation lath-arranged martensite phase through the martensite transformation zone. This final cooling step should be done quickly enough to avoid the formation of bainite, pearlite and precipitates along the interphase boundaries. The resulting microstructure consists of many grains bounded by an austenite shell, each grain having a univariate misaligned lath orientation rather than a multivariate orientation that would limit austenite stability. Alloy compositions suitable for use in the present invention are those capable of forming the dislocated lath structure by this type of processing. These compositions contain alloying elements in amounts selected to provide a martensite start temperature Ms of at least about 300°C, preferably at least about 350°C.

Brief description of the drawings

Figure 1 is a schematic diagram of the microstructure of a prior art alloy.

Figure 2 is a schematic diagram of the microstructure of the alloy of the present invention.

Detailed Description of Specific Embodiments

In order to be able to form the dislocation lath microstructure, the alloy composition must be an alloy composition whose Ms is about 300°C or above, preferably 350°C or above. Although alloying elements generally affect Ms, the alloying element that has the greatest influence on Ms is carbon, so limiting the carbon content in the alloy to a maximum of 0.35 wt. % can easily limit Ms to the desired range. In a preferred embodiment of the present invention, the carbon content is about 0.03-0.35%, and in a more preferred embodiment, the range is about 0.05-0.33%, all by weight.

It is also preferred to choose an alloy composition that avoids the formation of ferrite during the initial cooling of the alloy from the austenitic phase, i.e. before further cooling of the austenite to form the dislocated lath microstructure body grains. Also preferred are alloying elements comprising one or more austenite stabilizing elements such as carbon (which may already be included as described above), nitrogen, manganese, nickel, copper and zinc. Especially preferred austenite stabilizing elements are manganese and nickel. When nickel is present, it is preferably present at a concentration of about 0.25-5%, and when manganese is present, it is preferably at a concentration of about 0.25-6%. Chromium may also be included in many embodiments of the invention, preferably at a concentration of about 0.5-12%, when present. All concentrations stated herein are by weight. The presence and amount of each alloying element affects the martensitic onset temperature of the alloy. As noted above, alloys useful in the present invention are alloys having a martensitic onset temperature of at least about 350°C. Therefore, alloying elements and their amounts must be selected under this constraint. The alloying element that has the greatest influence on the martensite initiation temperature is carbon, and limiting the carbon content to a maximum of 0.35% generally ensures that the martensite initiation temperature is within the desired range. Other alloying elements such as molybdenum, titanium, niobium, and aluminum may also be present in amounts sufficient to serve as the nucleation sites required for the formation of fine grains, and in concentrations low enough that their presence does not affect the properties of the final alloy .

Preferred alloys of the invention are also substantially free of carbides. The term "substantially free of carbides" means that the distribution and amount of the precipitates are such that the carbides have little, if any, negative impact on the properties of the final alloy, especially the corrosion properties, if they actually exist. When carbides are present, they exist as precipitates embedded in the crystalline structure, and if the diameter of the precipitate is less than 500 Ȧ, its adverse effect on the properties of the alloy is minimal. It is important that there are no precipitates at the phase boundaries.

As mentioned above, reducing the grain size to 10 μm or less, can obtain martensite-austenite grains with univariate dislocation lath microstructure, that is, martensite lath and austenite grains inside each grain The bulk films are all oriented in the same orientation. The grain size is preferably about 1-10 microns, most preferably about 5-9 microns.

While the invention extends to alloys having the microstructure described above, regardless of the particular metallurgical method used to obtain said microstructure, certain specific processing steps are preferred. These preferred steps start with obtaining an alloy in the desired compositional proportions and then homogenize (hold) the alloy composition for a period of time at a temperature sufficient to obtain a homogeneous austenite with all elements and components in solid solution structure. The soak temperature is above the austenite crystallization temperature, which varies with the alloy composition, but is generally apparent to those skilled in the art. In most cases, the best effect can be obtained by keeping the temperature at 1050-1200°C. The alloy may also be rolled or forged or rolled and forged at this temperature.

After homogenization is complete, the alloy is cooled and the grains are refined to a desired grain size, eg, 10 microns or less, and preferably a narrower grain size range, as described above. The grain refinement can be carried out in multiple stages, but the final grain refinement is usually carried out at an intermediate temperature above but close to the austenite crystallization temperature. In this preferred method, the alloy is first rolled at the homogenization temperature (ie, undergoes dynamic recrystallization), then cooled to the intermediate temperature, and rolled again for further dynamic recrystallization. For the carbon steel alloys of the present invention, this intermediate temperature is between the austenite recrystallization temperature and a temperature about 50°C above the austenite recrystallization temperature. For the above preferred alloy composition, the austenite recrystallization temperature is about 900°C, therefore, the temperature to which the alloy is cooled at this stage is preferably about 900-950°C, most preferably about 900-925°C. Dynamic recrystallization can be achieved by conventional methods such as controlled rolling, forging, or a combination of both. The size reduction obtained by rolling is 10% or more, in many cases the size reduction is about 30-60%.

Once the desired grain size is achieved, the alloy is rapidly quenched by cooling from above the austenite recrystallization temperature to Ms and through the martensitic transition zone, transforming the austenite crystals into the dislocated cladding lath Microstructure. The size of the resulting sheath is substantially the same as the size of the austenite grains obtained in the rolling stage, but the residual austenite in these grains is in a thin film, in a shell around each grain. As mentioned above, the small grain size ensures that the grains are only univariate in the orientation of the austenite film.

As an alternative to dynamic recrystallization, grain refinement can be performed by two heat treatments, where the desired grain size is achieved by heat treatment alone. In this manner, as described in the previous paragraph, the alloy is quenched, then reheated to around or slightly below the austenite recrystallization temperature, and then quenched again to obtain or restore the dislocated lath microstructure. The reheating temperature is preferably about 50°C above the austenite recrystallization temperature, for example about 870°C.

In a preferred embodiment of the present invention, the quenching step of each of the above processes should be carried out at a sufficiently high cooling rate to avoid the formation of carbide precipitates, such as bainite and pearlite, and nitride and carbonitride precipitates (depending on the alloy composition) and avoid any precipitate formation at the phase boundaries. The terms "interphase precipitates" and "interphase precipitates" refer to precipitates at phase boundaries, and refer to the ) form small deposits of compounds. "Interphase precipitate" does not refer to the austenitic film itself. The formation of all of these types of precipitates, including bainite, pearlite, nitride and carbonitride precipitates, and interphase precipitates, is collectively referred to herein as "self-tempering".

The minimum cooling rate required to avoid self-tempering is evident from the deformation-temperature-time diagram of the alloy. In this graph, the vertical axis represents temperature and the horizontal axis represents time, and the curves in the figure represent regions where each phase exists by itself or mixed with other phases. This figure is found in Thomas US Patent No. 6,273,968 B1 cited above. In this figure, the minimum cooling rate is the slope of the temperature drop with respect to time, adjacent to the left of a C-shaped curve. The area to the right of the curve indicates the presence of carbides, therefore, the sloped line to the left of the curve indicates acceptable cooling rates, with the slowest rate having the smallest slope and adjoining the curve.

Depending on the alloy composition, a cooling rate sufficient to meet this requirement may be one that requires water cooling, or one that can be achieved by air cooling. Typically, alloy compositions that are air-coolable and still have a sufficiently high cooling rate, if the content of certain alloying elements are reduced, the content of other alloying elements must be increased to maintain the ability to use air cooling. For example, a decrease in one or more alloying elements such as carbon, chromium or silicon can be compensated by increasing the content of an element such as manganese. However, no matter how the individual alloying elements are adjusted, the final alloy composition must be one with a Ms greater than about 300°C, preferably greater than about 350°C.

The process steps and conditions described in the aforementioned U.S. patents can be used in the steps of the present invention of heating the alloy composition to the austenitic phase, cooling the alloy, and subjecting it to rolling or forging in a controlled manner to obtain the obtained desired size reduction and grain size, and quenching of the austenite grains through the martensitic transition zone to obtain the dislocation lath structure. These steps include casting, heat treatment of the alloy, and thermal working, such as forging or rolling at controlled temperatures for optimal grain refinement, finishing. Controlled rolling serves various functions, including facilitating the diffusion of alloying elements to form a uniform austenite crystal phase; and assisting in the storage of strain energy in the grains. During the quenching phase of the process, controlled rolling transforms the newly formed martensite phase into a dislocation-lath structure of martensite laths separated by thin films of retained austenite. The degree of reduction in rolling varies, as will be apparent to those skilled in the art. Quenching must be rapid enough to avoid the formation of bainite, pearlite and interphase precipitates. In martensite-austenite dislocation lath crystallization, the retained austenite film accounts for about 0.5-15% by volume of the microstructure, preferably about 3-10%, most preferably about 5%.

Comparing Figures 1 and 2, the difference between the present invention and the prior art can be seen. Figure 1 represents the prior art, showing a single grain 11 with a dislocated lath structure. The grains comprise four inner regions 12, 13, 14 and 15, each region consisting of martensitic dislocation laths 16 separated by austenite films 17, the orientation of the austenite films in each region being different from those in the remaining regions different orientations (that is, different variables). Consequently, adjacent regions are discontinuous in terms of the dislocated lath microstructure. Outside the grains is an austenite shell 18, although the boundaries between regions 19 (indicated by dashed lines) are not occupied by any precipitated discrete crystalline structure, but this simply represents the end of one variant while the other begins.

Figure 2 depicts two grains 21 and 22 of the present invention, each grain consisting of dislocated martensitic laths 23 separated by an austenite film 24, with only a single variable in terms of austenite film orientation, said The grains still have an austenitic shell 25 . The variation of one grain 21 is different from that of the other grain 22, but is only univariate within each grain.

The above description is mainly for the purpose of illustration. Modifications or changes may be made to the individual parameters of the alloy composition, process steps and conditions, while remaining within the basic and novel concept of the invention. These are obvious to those skilled in the art and are included within the scope of the present invention.

Claims (10)

1. An alloy carbon steel having a martensite initiation temperature of at least 300°C, comprising martensite-austenite grains 10 microns or less in diameter, each grain bounded by an austenite shell, and It has a uniformly oriented microstructure comprising martensite laths alternating with thin films of austenite throughout the grains. 2. The alloy carbon steel of claim 1, wherein the martensite initiation temperature is at least 350°C. 3. The alloyed carbon steel of claim 1, wherein the alloyed carbon steel contains at most 0.35% by weight of carbon. 4. The alloy carbon steel according to claim 1, characterized in that the martensite-austenite grain diameter is 1-10 microns. 5. The alloy carbon steel according to claim 1, characterized in that the alloy carbon steel further comprises 1-6% of an element selected from nickel and manganese. 6. The alloy carbon steel according to claim 1, characterized in that, all by weight, the alloy carbon steel comprises 0.05-0.33% carbon, 0.5-12% chromium, 0.25-5% nickel, 0.26-6% manganese and less than 1% silicon. 7. A method for manufacturing high-strength, high-corrosion-resistant tough alloy carbon steel, said method comprising: (a) Alloyed carbon steels with a martensite initiation temperature of at least 300°C; (b) heating the carbon steel alloy composition to a temperature high enough that the alloy composition forms a uniform austenite phase with all alloying elements in solid solution; (c) treating said homogeneous austenitic phase to a grain size of 10 microns or less when said austenitic phase is above its austenite recrystallization temperature; (d) cooling the austenite phase through the martensite transition zone, transforming the austenite phase into a microstructure of bound grains, each grain being 10 microns or less in diameter, and The grains contain martensite laths that alternate with retained austenite films in a uniform orientation. 8. The method of claim 7, wherein step (b) comprises heating said carbon steel alloy composition in the range of 1050-1200° C., said method further comprising heating said carbon steel alloy composition after step (b). The homogeneous austenitic phase is cooled to an intermediate temperature in the range of 900-950° C. at which part of said step (c) rolling is performed. 9. The method according to claim 7, characterized in that the grain size in the step (c) is 1-10 microns in diameter. 10. The method of claim 7, wherein, all by weight, the alloy carbon steel comprises 0.05-0.33% carbon, 2-12% chromium, 0.25-5% nickel, 0.26-6% manganese and 1 % of silicon below.

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